I need to prove that log_(10)2 is irrational. I understand the way this proof was done using contradiction to show that the even LHS does not equal the odd RHS, but I did it a different way and wanted to check its validity!

cdcamandokwd

cdcamandokwd

Answered question

2022-11-25

Alternate proof for " log 10 2 is irrational"
I need to prove that log 10 2 is irrational. I understand the way this proof was done using contradiction to show that the even LHS does not equal the odd RHS, but I did it a different way and wanted to check its validity!
Prove by contradiction: Suppose that log 2 is rational - that is, it can be written as
log 2 = a b
where a and b are integers. Then
2 = 10 a b
2 = 10 a 10 1 b
2 10 a = 10 1 b
Log both sides:
log ( 2 10 a ) = 1 b
log 2 log ( 10 a ) = 1 b
log 2 = 1 b + a
log 2 = a b + 1 b
However we assumed that log ( 2 ) = a b and thus we have a contradiction.

Answer & Explanation

Aileen Powers

Aileen Powers

Beginner2022-11-26Added 12 answers

As has been pointed out in comments and in another answer, 10 a / b 10 a 10 1 b . This is a rather subtle error, however there's a notable warning flag that could alert you to it: Your proof does not use the hypothesis that a and b are integers. This is a serious issue, because it means you've proved the (false) statement that log ( 2 ) cannot be written as a fraction a b - even if we let a and b be real, but:
log ( 2 ) 1 = log ( 2 )
Harmony Oneal

Harmony Oneal

Beginner2022-11-27Added 1 answers

A proof can be carried out after modifying your calculation a bit.
2 = 10 a b 2 b = 10 a 2 b a = 5 a
Which is a contradiction when both a and b are non-zero integers. Check the colored step carefully and you will understand in which step you have made a mistake.

Do you have a similar question?

Recalculate according to your conditions!

Ask your question.
Get an expert answer.

Let our experts help you. Answer in as fast as 15 minutes.

Didn't find what you were looking for?